Sometimes a customer brings me a
TV that I just can't fix. Maybe it's a faulty part for which
I can't get a replacement. Maybe it's a fault that I can't isolate
with certainty, and to buy say a microprocessor chip, would be
too big a gamble or would make the repair cost more than the
value of the set.
What really annoys me is the pricing strategy for some manufacturers' spare parts. Some suppliers, where there are no pattern parts, set their prices sky high. Then there's just no margin to charge a sensible sum for labour and still keep the repair viable. The other day a Hi-Fi needed four new parts. All the original pressure rollers had deteriorated, not through being used, but some sort of chemical reactions had made the rubber as hard as nails. The manufacturer quoted a price for one roller that would have made the repair expensive, but to replace all four was out of the question. Regrettably the job was declined. Although the rollers were worth much less than a pound, the cost of each from the manufacturer was over ten pounds.
The other day a large Goodmans TV arrived in the workshop with a symptom sounding like dry joints. It went off after a while but could be switched back on after a moment. Dry joints affect all TV sets sooner or later, in fact I've only recently taken the back off our family set and resoldered the main circuit board because ominous cracking noises had started to come from the loudspeaker.
The owner of the Goodmans, having
a technical bent had followed my instructions but had been unable
to sort out the problem. His set would go off from time to time:
it had done this for ages, but had got a lot worse lately, culminating
in it going off but not coming back on.
Under a strong light and a good magnifier dozens of cracking solder joints were revealed. All, bar one, were, in my opinion, just about to make their presence known. The rogue joint seemed to be in the base-band area.. in other words it was possible the picture had gone away intermittently leaving a blank raster. Yes this was true said the owner, and for some time, but this wasn't the current problem.
After a good resoldering exercise I switched on the TV. There was a familiar rustling noise from the EHT, bringing with it that feeling that all was well. Not for long however as within a few seconds the set went off with a whine.
I turned it off and thought for a moment.
While I was thinking, the owner then volunteered that he'd changed a resistor in the power supply that had looked frazzled. Maybe this hadn't been done properly and the HT was high, I wondered. I looked at the new resistor. It was actually two resistors soldered in parallel and they read 500 ohms. That seemed to fit in with what he'd said about the original.
I looked at the model number and went to look up the chassis. It was a Decca chassis, otherwise found under the Tatung brand name and it seemed to be a model 170 or 180. I fished out the U-View 1987-88 circuit compendium and soon found that it was a 180. The resistor should have been 500ohms so that was a red herring.
The design was pretty standard. A chopper power supply followed by a line output stage, which produced several other supply rails, including the one that powered the RF circuitry and the sound.
In the few seconds the set had been on there had been sound so the line output stage had been running. As a matter of course, almost without thinking, I'd checked the temperature of the metal plate behind the S2000AF transistor and found this to be very hot. In fact this had been so hot that the base-emitter junction was now reading a lot lower than it ought. I fitted a new one just to be safe. I then noticed that the clamping diode was dead short so in went a new BY228 as well.
My golden rule, which I don't always remember, is to look for distressed components before getting underway with random checks. Better late than never, I looked around the chassis and I noticed a distressed looking capacitor adjacent to the line output heat-sink. It was a 35 volt 1000uF component whose sleeve had shrunk leaving a clear eighth of an inch of can showing at the top. It's capacitance measured OK but to be safe, as its impedance was probably way too high, I fitted a nice new capacitor. Absolutely no improvement. The set came on for a moment and went off with a whine, taking out the new BY228.
Usually there are a number of common faults that can produce this effect. One is a faulty line output transformer. If this develops a leak between turns in the primary winding the stage will usually try to work but the output transistor driving the winding will get very hot because it will draw too much current.
Because this is so common the major manufacturer of these transformers sells a tester that can carry out a check of the various windings. The device emulates a TV set and a display shows the likely EHT value, or if there's a fault, a code indicating what's gone wrong. Typically, this can be a faulty primary winding or a short between windings.
I unsoldered the old transformer and fired up the workshop computer. Once I'd checked the pinning for the transformer from the CD that came with the tester I connected it and found that all was well.
Maybe there was a breakdown under the stress of the higher voltages in the TV compared with the tester so perhaps I shouldn't give the LOPT an absolute clean bill of health?
I decided to continue fault finding as there are other reasons for output transistor heating.
Maybe the scan coils have developed a short? Some of these use a glue that decomposes, eating away insulation until the coils short out and draw excessive current from the output transistor.
I replaced the transformer, disconnected the coils and tried the set. The same series of events happened again and the BY228 clamping diode bit the dust and went dead short. I fitted a replacement and plugged in the coils.
What about the HT line? When this goes high you can get this pattern of events. Sometimes there's a special zener diode arranged to go dead short if it sees say 10 volts more than the design value of HT. Not in this chassis though so I disconnected the line output stage and connected a 60 watt lamp. This lit up and the voltage across it measured exactly 150, corresponding to the value in the circuit diagram.
I removed the lamp and replaced all the connections and decided to check all the major components with my test meter. All the diodes were OK, resistors were fine and when I removed the four or five innocent-looking capacitors around the line output stage they also measured OK. That was unfortunate as the high voltage tuning capacitors can go low in value and blow up the line output transistor but these are simple to diagnose and replace.
Next I fished out my trusty book of TV stock faults. Nothing was listed other than things I'd already checked so I went to the workshop computer where there's a listing of all the faults in Practical TV from the year dot. A couple sounded interesting and I retrieved the relevant volumes from the filing cabinet in which resides copies from the last umpteen years.
One report sounded just like our fault. "Repeated failure of the transistor and clamping diode". This talked about the tuning capacitor though and these had measured OK. Another report talked about the E-W correction circuit and the final output transistor. I'd already checked this but just to be safe I removed it and tested it properly. My transistor tester proclaimed it was in fine shape so back it went.
What now? Well, when I'm at a loss as to what to do next, I usually wheel out the Variac and slowly crank up the mains input whilst making checks on the HT line etc. I connected everything up, and just to be ultra safe I wound down the HT pot in the chopper circuit. Was this a UK designed set? I mention this because the pot needed to be wound clockwise to reduce the voltage. I slowly cranked up the mains voltage. The HT settled at 135 volts. This set needs it's on/off switch to be pressed to kick it out of standby. I cranked down the mains to about 180 and pressed the on/off switch gently. The set rustled into life and the sound appeared. For more than a few seconds I thought it was OK then suddenly the sound went off and it whined at me. I disconnected the Variac and checked the output transistor and the diode. Both dead short. By now my stock of transistors and diodes was getting low.
I reckon it must be the transformer I muttered and looked it up. The price was around £20. That's a bit expensive I thought, especially as I imagined fitting it only to be rewarded with a few seconds of operation followed by a whine. In the past I'd discovered that this premonition invariably comes true so I shelved the idea of a new transformer until I was really sure.
The next day I decided that logic and proper fault finding should be the order of the day. I swept litter from the bench, which was now getting reasonably clear since an adjacent 33 inch monster had just been collected.
I removed the LOPT, fitted a new output transistor and a new diode, and prepared a number of lengths of wire. I then connected the transformer coils to the circuit board one by one. First the primary winding. I switched on the set. Nothing happened. At least there wasn't a whine and the HT measured 135. I reset it to 150 and carried on so far so good and without secondary connections I hadn't expected to get sound. I connected the CRT heater winding and switched on. Still nothing untoward. Next I connected the over-winding of the primary still OK. Things were looking up. In this set there were only a couple of secondary windings so I connected the last and the set came on briefly then whined.
There was the answer. The fault lay in the winding for the RF and sound circuitry.
What was the link to the line output stage though? It can't be an overload from the secondary circuit because I'd heard the sound build up unless it was some sort of thermal runaway, say in the audio output stage?
I examined the circuit diagram. There was a 1mH choke connecting the secondary winding to the line output and scan coils circuit.
I removed it. It looked pristine but I've had these things develop an internal short in the past and a few times I remembered that the line output transistor had got very hot because of this. I rigged up a test circuit and made some measurements. After a few calculations the value of the thing worked out to be precisely 0.95mH. Too close to be faulty I thought, however I left it out and tried the set.
On came the sound and on came the picture. True it was too bright but that was just the brightness setting. Of course there was no E-W correction and if I'd had the handset this could have been seen from the text picture which would have had an egg-timer shape.
I connected a pair of wires to the circuit and briefly connected the choke. The sound and picture disappeared and there was whine but quickly disconnecting it restored both.
I turned off the set and studied the circuit diagram. The E-W circuit surely couldn't be the culprit? I checked the few transistors, resistors and diodes and all claimed to be entirely innocent. What else? The transformer secondary winding fed the RF and sound via a diode rectifier but it also fed raw AC to the scan circuit via the 1mH choke. This to provide pulses at the line frequency as part of E-W correction and width control but this part of the scan circuit is isolated from ground by capacitors and surely would not permit what seemed to be a short circuit. These capacitors had all measured correctly so the fault must be at the other side of the choke. Here there was only the rectifier. This was a BA157 and I found it sitting next to the line output heat-sink, next to that distressed capacitor. It measured just like any other diode but on close examination, admittedly looked a little brown.
I removed it and fitted a new BA159, a similar device. Why was the rating way above 1kV when it served to produce only 30 volts?
Well I suppose the line output stage may develop that sort of pulse and the scan circuit is connected to the secondary winding. Maybe the high voltage pulses from the scan area appear at the anode of the diode?
I turned on the set. Up came the sound and up came the picture, this time with E-W correction!
The diode had been breaking down but in a "recoverable" way. This sort of failure is an "avalanche" type. In other words the diode conducts like a short piece of wire in the wrong direction but fully recovers once the trigger signal has been removed, something like a zener diode. Apart from the slight discolouration that should have indicated something may have been wrong, it's just like an ordinary diode when asked to do its primary job of rectifying 30 volts. Hit it with high voltage pulses and it becomes a virtual short circuit. At its cathode there was the 1000uF electrolytic, whose plastic sleeve, shrunk with past heating, clearly signalling that something was amiss. The fault had been making itself a nuisance, it turned out, for a long time. Each time it had happened the diode's breakdown voltage had been getting lower and lower until eventually it had given up each time the scan pulses had stabilised and had plonked AC on the capacitor making it very hot.
After removing all the temporary connecting wires from the LOPT I refitted it and tested the set. After adjusting the E-W and the width the picture was perfect.
I looked at the bench and collected the debris. There were three duff line output transistors and five duff BY228s, an innocent looking BA157 and a sorry looking 1000uF capacitor. At least I hadn't bought a new line output transformer!
The moral of the story is to pay more attention to the evidence. For instance.. why had the capacitor been getting hot, for surely it must have been getting very hot indeed to shrink its plastic sleeve?